Efficient Photocatalytic Fuel Cell via Simultaneous Visible-Photoelectrocatalytic Degradation and Electricity Generation on a Porous Coral-like WO3/W Photoelectrode

Approaches for Enhancing Wastewater Treatment of Photocatalytic Fuel Cells: A Review

Environmental pollution and energy crises have garnered global attention. The substantial discharge of organic waste into water bodies has led to profound environmental contamination. Photocatalytic fuel cells (PFCs) enabling the simultaneous removal of refractory contaminants and recovery of the chemical energy contained in organic pollutants provides a potential strategy to solve environmental issues and the energy crisis. This review will discuss the fundamentals, working principle, and configuration development of PFCs and photocatalytic microbial fuel cells (PMFCs). We particularly focus on the strategies for improving the wastewater treatment performance of PFCs/PMFCs in terms of coupled advanced oxidation processes, the rational design of high-efficiency electrodes, and the strengthening of the mass transfer process. The significant potential of PFCs/PMFCs in various fields is further discussed in detail. This review is intended to provide some guidance for the better implementation and widespread adoption of PFC wastewater treatment technologies.

A photoelectrochemical aptasensor for omethoate determination based on a photocatalysis of CeO2@MnO2 heterojunction for glucose oxidation

In the present work, we developed a photoelectrochemical aptasensor to determine omethoate (OMT) based on the dual signal amplification of CeO2@MnO2 photocatalysis for glucose oxidation and exonuclease I-assisted cyclic catalytic hydrolysis. CeO2@MnO2 heterojunction material prepared by hydrothermal method was linked with captured DNA (cDNA) and then assembled on the ITO conductive glass to form ITO/CeO2@MnO2-cDNA, which exhibited significant photocurrent response and good photocatalytic performance for glucose oxidation under visible light irradiation, providing the feasibility for sensitive determining OMT. After binding with the aptamer of OMT (apt), the formation of rigid double stranded cDNA/apt kept CeO2@MnO2 away from ITO surface, which ensured a low photocurrent background for the constructed ITO/CeO2@MnO2-cDNA/apt aptasensor. In the presence of target OMT, the restoration of the cDNA hairpin structure and the exonuclease I-assisted cyclic catalytic hydrolysis led to the generation and amplification of measurement photocurrent signals, and allowed the aptasensor to have an ideal quantitative range of 0.01-10.0 nM and low detection limit of 0.0027 nM. Moreover, the aptasensor has been applied for selective determination of OMT in real samples with good precision of the relative standard deviation less than 6.2 % and good accuracy of the recoveries from 93 % to 108 %. What's more, the aptasensor can be used for other target determination only by replacing the captured DNA and corresponding aptamer.

Photo-self-Fenton Reaction Mediated by Atomically Dispersed Ag-Co Photocatalysts toward Efficient Degradation of Organic Pollutants

Achieving the complete mineralization of persistent pollutants in wastewater is still a big challenge. Here, we propose an efficient photo-self-Fenton reaction for the degradation of different pollutants using the high-density (Ag: 22 wt %) of atomically dispersed AgCo dual sites embedded in graphic carbon nitride (AgCo-CN). Comprehensive experimental measurements and density functional theory (DFT) calculations demonstrate that the Ag and Co dual sites in AgCo-CN play a critical role in accelerating the photoinduced charge separation and forming the self-Fenton redox centers, respectively. The bimetallic AgCo-CN exhibited excellent photocatalytic performance toward the phenol even under extreme conditions due to an efficient degradation pathway and in situ generation of the hydrogen peroxide producing the main active oxygen species (⋅OH and 1 O2 ) and showed long-term activity in a self-design photo-Filter reactor for the purification of the phenol. Our discoveries pave the way for the design of efficient single-atoms photocatalysts-based photo-self-Fenton reaction for recalcitrant pollutant treatment.

Three dimensional nickel foam carried sea urchin-like copper-cobalt-cerium cathode for enhanced tetracycline wastewater purification in photocatalytic fuel cell

Photocatalytic fuel cells (PFCs) regarded as a potential sustainable technique, have been broadly reported. In this work, the carbon quantum dot-loaded TiO2 photoanode and sea urchin-like CuCoCe ternary metal oxide cathode materials are successfully synthesized and used to construct PFC systems for efficient tetracycline (TC) degradation (45 mg/L) and simultaneous electricity generation. The results demonstrate that the CQDs-modified TiO2 photoanode has improved absorption intensity in both the UV and visible regions, and the photocurrent density at 1.23 V vs RHE reached 1.31 mA cm-2, which is 1.3 times higher than that of the original TiO2 photoanode. The established PFC system achieves the highest removal ratio of 96.9 % for TC in 60 min with a maximum power density of 0.77 mW cm-2. The PFC system can operate efficiently over a wide pH range (3.0-9.0). Furthermore, quenching experiments and ESR spectra show that the main reactive oxygen species in the degradation process are •O2-, 1O2 and •OH. This study provides meaningful way to develop multiple metal oxides as cathode of PFC system for efficient organic pollutant degradation and energy recovery.

Sustainable degradation of pollutants, generation of electricity and hydrogen evolution via photocatalytic fuel cells: An Inclusive Review

Environmental pollution and energy crisis have recently become one of the major global concerns. Insincere discharge of massive amount of organic and inorganic wastes into the aqueous bodies causes serious impact on our environment. However, these organic substances are significant sources of carbon and energy that could be sustainably utilized rather than being discarded. Photocatalytic fuel cell (PFC) is a smart and novel energy conversion device that has the ability to achieve dual benefits: degrading the organic contaminants and simultaneously generating electricity, thereby helping in environmental remediation. This article presents a detailed study of the recent advancements in the development of PFC systems and focuses on the fundamental working principles of PFCs. The degradation of various common organic and inorganic contaminants including dyes and antibiotics with simultaneous power generation and hydrogen evolution has been outlined. The impact of various operational factors on the PFC activity has also been briefly discussed. Moreover, it provides an overview of the design guidelines of the different PFC systems that has been developed recently. It also includes a mention of the materials employed for the construction of the photo electrodes and highlights the major limitations and relevant research scopes that are anticipated to be of interest in the days to come. The review is intended to serve as a handy resource for researchers and budding scientists opting to work in this area of PFC devices.

Treatment of diluted palm oil mill effluent (POME) synchronous with electricity production in a persulfate oxidant-promoted photocatalytic fuel cell

Attributable to the prosperous production growth of palm oil in Malaysia, the generated palm oil mill effluent (POME) poses a high threat owing to its highly polluted characteristic. Urged by the escalating concern of environmental conservation, POME pollution abatement and potential energy recovery from the effluent are flagged up as a research topic of interest. In this study, a cutting-edge photocatalytic fuel cell (PFC) system with employment of ZnO/Zn nanorod array (NRA) photoanode, CuO/Cu cathode, and persulfate (PS) oxidant was successfully designed to improve the treatment of POME and simultaneous energy production. The photoelectrodes were fabricated and characterized by field emission scanning electron microscopy with energy (FESEM), X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and Brunauer, Emmett, and Teller analysis (BET). Owing to the properties of strong oxidant of PS, the proposed PFC/PS system has exhibited exceptional performance, attaining chemical oxygen demand (COD) removal efficiency of 96.2%, open circuit voltage (Voc) of 740.0 mV, short circuit current density (Jsc) of 146.7 μA cm-2, and power density (Pmax) of 35.6 μW cm-2. The pre-eminent PFC/PS system performance was yielded under optimal conditions of 2.5 mM of persulfate oxidant, POME dilution factor of 1:20, and natural solution pH of 8.51. Subsequently, the postulated photoelectrocatalytic POME treatment mechanism was elucidated by the radical scavenging study and Mott-Schottky (M-S) analysis. The following recycling test affirmed the stability and durability of the photoanode after four continuous repetition usages while the assessed electrical energy efficiency revealed the economic viability of PFC system serving as a post-treatment for abatement of POME. These findings contributed toward enhancing the sustainability criteria and economic viability of palm oil by adopting sustainable and efficient POME post-treatment technology.

Activation of chloride by oxygen vacancies-enriched TiO2 photoanode for efficient photoelectrochemical treatment of persistent organic pollutants and simultaneous H2 generation

Photoelectrochemical (PEC) activation of chloride ions (Cl^-) to degrade persistent organic pollutants (POPs) is a promising strategy for the treatment of industrial saline organic wastewater. However, the wide application of this technology is greatly restricted due to the general photoanode activation of Cl^- with poor capability, the propensity to produce toxic by-products chlorates, and the narrow pH range. Herein, oxygen vacancies-enriched titanium dioxide (O_v-TiO₂) photoanode is explored to strongly activate Cl^- to drive the deep mineralization of POPs wastewater in a wide pH range (2-12) with simultaneous production of H₂. More importantly, nearly no toxic by-product of chlorates was produced during such PEC-Cl system. The degradation efficiency of 4-CP and H₂ generation rate by O_v-TiO₂ were 99.9% within 60 min and 198.2 μmol h^-1 cm^-2, respectively, which are far superior to that on the TiO₂ (33.1% within 60 min, 27.5 μmol h^-1 cm^-2) working electrode. DFT calculation and capture experiments revealed that O_v-TiO₂ with abundant oxygen vacancies is conducive to the activation of Cl^- to produce more reactive chlorine species, evidenced by its high production of free chlorine (48.7 mg L^-1 vs 7.5 mg L^-1 of TiO₂). The as-designed PEC-Cl system in this work is expected to realize the purification of industrial saline organic wastewater coupling with green energy H₂ evolution.

Efficient Bias-Free Degradation of Sulfamethazine by TiO2 Nanoneedle Arrays Photoanode and Co3O4 Photocathode System under LED-Light Irradiation

Solving high electrical-energy input for pollutants degradation is one of the core requirements for the practical application of photoelectrocatalytic (PEC) technology. Herein, we developed a self-driven dual-photoelectrode PEC system (TiO2 NNs-Co3O4) composed of a TiO2 nanoneedle arrays (TiO2 NNs) photoanode and Co3O4 photocathode for the first time. Under light-emitting-diode (LED) illumination, the bias-free TiO2 NNs-Co3O4 PEC system exhibited excellent PEC performance, with an internal bias as high as 0.19 V, achieving near complete degradation (99.62%) of sulfamethazine (SMT) with a pseudo-first-order rate constant of 0.042 min−1. The influences of solution pH, typical inorganic anions, natural organic matter, and initial SMT concentration on the PEC performance were investigated. Moreover, the main reactive oxygen species (h+, •OH, •O2−) in the dual-photoelectrode PEC system for SMT decomposition were elaborated. The practical application feasibility for efficient water purification of this unbiased PEC system was evaluated. It was proved that the TiO2 NNs photoanode provided a negative bias while the Co3O4 photocathode provided a positive bias for the photoanode, which made this system operate without external bias. This work elucidated the cooperative mechanism of photoelectrodes, providing guidance to develop a sustainable, efficient, and energy-saving PEC system for wastewater treatment.

UV-Visible-Near-Infrared-Driven Photoelectrocatalytic Urea Oxidation and Photocatalytic Urea Fuel Cells Based on Ruddlensden–Popper-Type Perovskite Oxide La2NiO4

Photocatalysis and photoelectrocatalysis, as green and low-cost pollutant treatment technologies, have been widely used to simultaneously degrade pollutants and produce clean energy to solve the problems of environmental pollution and energy crisis. However, the disadvantages of photocatalysts in a narrow absorption range and low utilization rate of solar energy still hinder the practical application. Here we fabricate two-dimensional porous Ruddlensden–Popper type nickel-based perovskite oxide La2NiO4 as a noble metal-free photoanode for photoelectrocatalytic urea oxidation under full spectrum sunlight irradiation. The transient photocurrent density under near infrared (NIR) light (λ > 800 nm) can reach 50 μA cm−2. Urea wastewater was used as the fuel to obtain low-energy hydrogen production, and round-the-clock hydrogen production was achieved with the optimal yield of 22.76 μmol cm−2 h−1. Moreover, a photocatalytic urea fuel cell (PUFC) was constructed with La2NiO4 as the photoanode. The power density under UV-vis-NIR was 0.575 μW cm−2. Surprisingly, the filling factor (FF) under NIR light was 0.477, which was much higher than those under UV-vis-NIR and visible light. The results demonstrated that PUFCs constructed from low-cost nickel-based perovskite oxides have potential applications for low-energy hydrogen production and efficient utilization of sunlight.

Laccase-coupled photoelectrocatalytic system for highly efficient degradation of bisphenol A

In the present work, a hybrid advanced oxidation process that combined laccase with photoelectrocatalysis (PEC) was explored for highly efficient degradation of bisphenol A (BPA). Visible light-responsive BiVO₄ film electrode with good optical and photoelectrochemical properties was prepared via an electrodeposition method and employed as photoanode for PEC degradation of BPA. After laccase was facilely introduced into the PEC system, the BPA removal efficiency was significantly promoted, attributed to the synergistic effect of enzymatic catalysis and PEC processes. To obtain the optimum operation conditions, the effects of initial pH and applied bias potential were investigated systematically. Radicals trapping experiments revealed that •O₂^- dominated the biophotoelectrocatalytic degradation process, and the possible degradation pathway for BPA was proposed by identifying intermediates using liquid chromatography-mass spectrometry.

Synergy of photocatalysis and fuel cells: A chronological review on efficient designs, potential materials and emerging applications

The rising demand of energy and lack of clean water are two major concerns of modern world. Renewable energy sources are the only way out in order to provide energy in a sustainable manner for the ever-increasing demands of the society. A renewable energy source which can also provide clean water will be of immense interest and that is where Photocatalytic Fuel Cells (PFCs) exactly fit in. PFCs hold the ability to produce electric power with simultaneous photocatalytic degradation of pollutants on exposure to light. Different strategies, including conventional Photoelectrochemical cell design, have been technically upgraded to exploit the advantage of PFCs and to widen their applicability. Parallel to the research on design, researchers have put an immense effort into developing materials/composites for electrodes and their unique properties. The efficient strategies and potential materials have opened up a new horizon of applications for PFCs. Recent research reports reveal this persistently broadening arena which includes hydrogen and hydrogen peroxide generation, carbon dioxide and heavy metal reduction and even sensor applications. The review reported here consolidates all the aspects of various design strategies, materials and applications of PFCs. The review provides an overall understanding of PFC systems, which possess the potential to be a marvellous renewable source of energy with a handful of simultaneous applications. The review is a read to the scientific community and early researchers interested in working on PFC systems.

Nitrate promoted defluorination of perfluorooctanoic acid in UV/sulfite system: Coupling hydrated electron/reactive nitrogen species-mediated reduction and oxidation

A significantly accelerated defluorination of recalcitrant perfluorooctanoic acid (PFOA) was explored with the co-present nitrate (20 mg L^-1) by UV/sulfite treatment (UV/sulfite-nitrate). The deep defluorination of PFOA and complete denitrification of nitrate were simultaneously achieved in UV/sulfite-nitrate system. At the initial 30 min, PFOA defluorination exhibited an induction period, exactly corresponding to the removal of the co-existed nitrate. Upon the induction period passed, an accelerated removal of PFOA (5 mg L^-1) occurred, nearly 100% defluorination ratio reached within 2 h. Compared with those in UV/sulfite, the kinetics of PFOA decay, defluorination, and transformation product formations were greatly enhanced in UV/sulfite-nitrate system. Reactive nitrogen species (RNS) generated from e_aq^--induced reduction of nitrate were found to play significant roles on the promoted defluorination apart from e_aq^--mediated reductive defluorination. The investigations on solution pH (7.0-11.0) confirmed that the reductive defluorination of PFOA was more efficient under alkaline conditions, however, the presence of nitrate can promote the defluorination even under neutral pH. Theoretical calculations of Fukui function demonstrated that RNS could easily launch electrophilic attack toward H-rich moieties of fluorotelomer carboxylates (FTCAs, C_nF_2n+1-(CH₂)_m-COO^-), more persistent intermediates (formed via H/F exchange), and convert FTCAs into shorter-chain perfluorinated carboxylic acids, thus facilitating the deep defluorination. Along with the analysis on the denitrification products, the liberation of fluoride ions and generated intermediates, possible decomposition pathways were proposed. This work highlights the indispensable synergy from e_aq^-/RNS with integrated reduction and oxidation on PFOA defluorination and will advance remediation technologies of perfluorinated compound contaminated water.

Singlet Oxygen and Mobile Hydroxyl Radicals Co-operating on Gas-Solid Catalytic Reaction Interfaces for Deeply Oxidizing NOx

Learning from the important role of porphyrin-based chromophores in natural photosynthesis, a bionic photocatalytic system based on tetrakis (4-carboxyphenyl) porphyrin-coupled TiO₂ was designed for photo-induced treating low-concentration NO_x indoor gas (550 parts per billion), achieving a high NO removal rate of 91% and a long stability under visible-light (λ ≥ 420 nm) irradiation. Besides the great contribution of the conventional ^•O₂^- reactive species, a synergic effect between a singlet oxygen (¹O₂) and mobile hydroxyl radicals (^•OH_f) was first illustrated for removing NO_x indoor gas (¹O₂ + 2NO → 2NO₂, NO₂ + ^•OH_f → HNO₃), inhibiting the production of the byproducts of NO₂. This work is helpful for understanding the surface mechanism of photocatalytic NO_x oxidation and provides a new perspective for the development of highly efficient air purification systems.

Efficient removal of PFOA with an In2O3/persulfate system under solar light via the combined process of surface radicals and photogenerated holes

The environmental persistence, high toxicity and wide spread presence of perfluorooctanoic acid (PFOA) in aquatic environment urgently necessitate the development of advanced technologies to eliminate PFOA. Here, the simultaneous application of a heterogeneous In₂O₃ photocatalyst and homogeneous persulfate oxidation (In₂O₃/PS) was demonstrated for PFOA degradation under solar light irradiation. The synergistic effect of direct hole oxidation and in-situ generated radicals, especially surface radicals, was found to contribute significantly to PFOA defluorination. Fourier infrared transform (FTIR) spectroscopy, Raman, electrochemical scanning microscope (SECM) tests and density functional theory (DFT) calculation showed that the pre-adsorption of PFOA and PS onto In₂O₃ surface were dramatically critical steps, which could efficiently facilitate the direct hole oxidation of PFOA, and boost PS activation to yield high surface-confined radicals, thus prompting PFOA degradation. Response surface methodology (RSM) was applied to regulate the operation parameters for PFOA defluorination. Outstanding PFOA decomposition (98.6%) and near-stoichiometric equivalents of fluorides release were achieved within illumination 10 h. An underlying mechanism for PFOA destruction was proposed via a stepwise losing CF₂ unit. The In₂O₃/PS remediation system under solar light provides an economical, sustainable and environmentally friendly approach for complete mineralization of PFOA, displaying a promising potential for treatment of PFOA-containing water.

Complete solar-driven dual-photoelectrode fuel cell for water purification and power generation in the presence of peroxymonosulfate

This study reports the development of complete solar-driven dual-photoelectrode fuel cell (PFC) based on WO₃ photoanode and Cu₂O photocathode with peroxymonosulfate (PMS) serving as cathodic electron acceptor. As indicated by photoelectrochemical measurements, the PMS was able to improve thermodynamic properties of photocathode, achieving an increased open circuit potential from 0.42 V to 0.65 V vs standard hydrogen electrode (SHE). Under simulated sunlight irradiation (~100 mW cm^-2), the maximum power density of 0.12 mW cm^-2 could be obtained at current density of 0.34 mA cm^-2, which was 8.57 times of that produced by PFC without PMS (0.014 mW cm^-2). Correspondingly, adding PMS (1.0 mM) increased overall removal efficiency of 4-chlorophenol (4-CP) from 39.8% to 96.8%, accounting for the first-order kinetic constant (k=0.056 min^-1) being 6.67 times of that in the absence of PMS (k=0.0084 min^-1). Radical quenching and electron spin-resonance (ESR) results suggested the contribution of free radicals (•OH and SO₄^•-) and non-radical pathway associated with direct activation of PMS by Cu₂O photocathode. Fourier transformed infrared (FTIR) analysis confirmed the strong non-radical interaction between Cu₂O photocathode and PMS, resulting in 4-CP removal via activation of PMS by surface complex on Cu₂O. The proof-in-concept complete solar-driven dual-photoelectrode fuel cell may offer an effective manner to realize water purification and power generation, making wastewater treatment more economical and more sustainable.

Modification of Black Phosphorus Nanosheets with a Ni-Containing Carbon Layer as Efficient and Stable Hydrogen Production Electrocatalysts

Few-layered black phosphorus (FP) has recently attracted extensive research in the energy and materials fields. However, because of its chemically unstable nature under ambient conditions, very positive hydrogen adsorption energy and less active sites, FP has not been an efficient catalyst for the hydrogen evolution reaction (HER). In this research, we have developed a new strategy to overcome FP's drawbacks and to make it an active and stable HER catalyst. Our approach is to deposit a Ni²⁺-anchored thin carbon layer onto the surface of FP via controlled decarboxylation of Ni ethylenediaminetetraacetate (Ni-EDTA). The carbon layer on the surface of FP prevents it from making direct contact with its external environment, thereby greatly improving its stability. At the same time, transition-metal Ni that is dispersed in its carbon layer changes its hydrogen adsorption energy so as to improve its electrocatalytic activity. The prepared FP@Ni-C shows an outstanding HER performance with an overpotential of only 284 mV to obtain 10 mA cm^-2 current density with excellent electrocatalytic stability. The FP@Ni-C catalyst showed almost no activity loss during a 12 h catalyst life test. This study provides a new approach to the synthesis of highly efficient and stable electrocatalysts based on two-dimensional materials, using a facile catalyst preparation method.

Efficient Self-Driving Photoelectrocatalytic Reactor for Synergistic Water Purification and H2 Evolution

The photoelectrocatalytic (PEC) technique has attracted much attention to getting clear energy and environmental purification. Simultaneous reactions of solar energy generation could be used to apply for practical applications to maximize the functionality of reactor systems. Herein, we crafted a self-driving photoelectrocatalytic reactor system, comprising platinum (Pt) modified p-Si nanowires (Pt/Si-NWs) as a photocathode and TiO₂ nanotube arrays (TiO₂-NTAs) as a photoanode for synergistic H₂ evolution and water purification, respectively. Hydrogen evolution in the cathode chamber and environmental remediation in the anode chamber were achieved with the aid of appropriate bandgap illumination and self-built bias voltage. The mismatch of Fermi levels between TiO₂-NTAs and Si-NWs reduced the recombination rates of photoinduced electrons and holes through the formation of Z scheme and inner electric filed. The synergistic PEC reactions exhibited much higher activities than those achieved using other systems so far. This basic principal could be applied for fabricating other PEC reactors in photoelectro conversion devices and be established as design guidelines for reactors to maximize the PEC performance.

Solid-Phase Microwave Reduction of WO3 by GO for Enhanced Synergistic Photo-Fenton Catalytic Degradation of Bisphenol A

The synergistic photocatalytic Fenton reaction is a powerful advanced oxidation technique for the degradation of persistent organic pollutants. However, microwave-induced thermal effects on the formation of novel structures facilitating the photocatalytic degradation have been rarely reported. Herein, a two-step microwave thermal strategy was developed to synthesize a new hybrid catalyst comprising defective WO_3-x nanowires coupled with reduced graphene oxides (rGOs). Conventionally, microwave methods could induce superhot spots on the GO surface, resulting in the site-specific crystallization and oriented growth of WO₃. However, in the solid phase, localized microwave thermal effects could reduce the interfacial area between WO₃ and rGO and enhance the bonding between them. As for the unique structure and surface properties, the synthesized catalyst enhanced the light absorption, promoted the interfacial charge separation, and increased the carrier density in the photocatalytic processes. In addition, surface formation of W⁴⁺ provided a new pathway for Fe³⁺/Fe²⁺ cycling which linked the photocatalytic reaction and the Fenton process. The optimized catalyst exhibited a remarkable performance in the degradation of bisphenol A with a ∼83% removal yield via a photo-Fenton route. These microwave-induced functionalities of materials for synergistic reactions could also give a new avenue to other photoelectrocatalytic fields and solar cells.

Solar driven self-sustainable photoelectrochemical bacteria inactivation in scale-up reactor utilizing large-scale fabricable Ti/MoS2/MoOx photoanode

Here we present photoelectrochemical (PEC) bacterial inactivation properties of large-scale fabricable Ti/MoS₂/MoO_x photoanode with a strong solar light absorbance capacity. Specifically, by thermal oxidation of the as-prepared MoS₂/Ti film at 250 °C for 15 min in aerobic condition, the visible light performance of photocurrent generation and Escherichia coli (E. coli) inactivation are markedly enhanced. Complete inactivation of 10⁶ CFU/mL E. coli in NaCl electrolyte is achieved with 0.5 V bias in 2 h under visible light irradiation, and H₂O₂ and O₂^- have been found as key reactive oxidative species to destroy E. coli. The bacteria inactivation performance of present photoanode is comparable with reported visible light photoanodes such as Cu₂O or N-doped TiO₂. The markedly improved PEC performance and inhibited photocorrosion could be attributed to the formation of heterojunction of MoS₂/MoO_x on the surface due to thermal oxidation. Furthermore, the PEC E. coli inactivation performance and stability of the large dimensional electrode are evaluated in a scale-up reactor. As an example of self-sustainable PEC water treatment system powered by only solar panels, wastewater containing inorganic, organic, macromolecule and microbial pollutants is attempted to be treated employing the developed electrodes under illumination of LED lamps.

Efficient organic pollutants conversion and electricity generation for carbonate-containing wastewater based on carbonate radical reactions initiated by BiVO4-Au/PVC system

In this paper, we propose an efficient simultaneous refractory organics degradation and electricity generation method for carbonate-containing wastewater based on carbonate radical reactions initiated by a BiVO₄-Au/PVC (PVC: photovoltaic cell) system. In the system, nanoporous BiVO₄ film and Au modified PVC were used as photoanode and photocathode, respectively. HCO₃^- was used as the electrolyte. Carbonate radicals, which have lower recombination rates than hydroxyl radicals and strong oxidation abilities, can be generated easily by the capture reaction of hydroxyl radicals with HCO₃^-, which is one of the most abundant anions in the aquatic environment. The results show that the removal ratios of rhodamine b, methyl orange and methylene blue in the system increased sharply to 77.98 %, 89.15 % and 93.2 % from 18.23 %, 21 % and 23.14 % (BiVO₄-Pt/ SO₄^2-), respectively, after 120 min. Meanwhile, the short-circuit current density is up to 2.19-2.41 times larger than the traditional system. Other common ions in natural water minimally affected the properties of the new system. The excellent performance could be ascribed to large amounts of carbonate radicals in the system, which have great potential for efficient carbonate-containing wastewater treatment and energy recovery.

Rapid photochemical decomposition of perfluorooctanoic acid mediated by a comprehensive effect of nitrogen dioxide radicals and Fe3+/Fe2+ redox cycle

Developing efficient methods to degrade perfluorochemicals (PFCs), an emerging class of highly recalcitrant contaminants, are urgently needed in recent years, due to their persistence, high toxicity, and resistance to most regular treatment procedures. Here, a UV-photolysis system is reported for efficient mineralization of perfluorooctanoic acid (PFOA) via irradiation of ferric nitrate aqueous solution, where in-situ generating •NO₂ and the effective Fe³⁺/Fe²⁺ redox cycle synergistically play great roles on rapidly mediating the mineralization of PFOA. A fast PFOA removal kinetics with first-order kinetic constants of 2.262 h^-1 is observed at initial PFOA concentration of 5 ppm (50 mL volume), reaching ∼ 92 % removal efficiency within only 0.5-h irradiation. Near-stoichiometric fluoride ions liberation and high total organic carbon (TOC) removal efficiency (∼100 %) further validated the capability for completely destructive removal of PFOA. A tentative pathway for PFOA destruction is proposed. This work, by UV photolysis of abundant existing iron/nitrate-based systems in natural environment, provides an economical, sustainable and highly efficient approach for complete mineralization of perfluorinated chemicals.

Exhaustive denitrification via chlorine oxide radical reactions for urea based on a novel photoelectrochemical cell

Urea is a major source of nitrogen pollution in domestic sewage and its denitrification is difficult since it is very likely to be converted into ammonia or nitrate instead of expected N₂. Herein, we propose an exhaustive denitrification method for urea via the oxidation of amine/ammonia-N with chlorine oxide radical, which induced from a bi-functional RuO₂//WO₃ anode, and the highly selective reduction of nitrate-N on cathode in photoelectrochemical cell (PEC). Under illumination, the WO₃ photoanode side promotes the quantities hydroxyl and reactive chlorine radical, and these radicals are immediately combined to stronger chlorine oxide radical by RuO₂ side, which obviously enhances the efficiency and speed of the urea oxidation. Synchronously, the over-oxidized nitrate can be selectively reduced by Pd and Au nanoparticles on the surface of cathode. Eventually, exhaustive denitrification is realized by the circulative reaction. Experimental observations and theoretical calculation revealed that chlorine oxide radical promoted significant denitrification of urea with an efficiency of 99.74% in 60 min under the optimum condition. The removal rate constant of the RuO₂//WO₃ anode was 3.08 times than that of single WO₃ anode and 2.64 times than that of single RuO₂ anode, confirming the chlorine oxide radical had stronger ability on denitrification than reactive chlorine radical. Also, the bi-functional anode contributed to best current efficiencies, utilizing the energy availably. This work proposes a promising method of exhaustive denitrification for urea.

Highly Efficient Hydrogen and Electricity Production Combined with Degradation of Organics Based on a Novel Solar Water-Energy Nexus System

A novel unassisted solar water-energy nexus system (SWENS) comprised of a monolithic photoanode, which was assembled by attaching a silicon cell (SC) at the reverse side of a high-activity hyaline antimony-doped TiO₂ nanorod array (Sb/TNR), and a Pt-black/Pt cathode was proposed for effective electricity and hydrogen production accompanying water treatment. The Sb/TNR with vertically arranged nanorods on a F-doped SnO₂ substrate, using a simple hydrothermal method, showed an excellently enhanced and stable photo-to-current density of ∼1.77 mA cm^-2 (0.6 V vs Ag/AgCl), which is ∼181% that of the undoped sample because antimony doping enhanced the charge-transfer property and charge-carrier density of Sb/TNR. The SWENS showed a removal ratio of nearly 100% for 2-chlorophenol after 4 h of operation under AM 1.5 illumination and achieved an average H₂ production rate of 31.4 μmol h^-1 cm^-2, an excellent electricity output with an open-circuit voltage of ∼2.16 V, a short-circuit current of ∼1857 μA cm^-2, and a maximum power output of ∼967 μW cm^-2, which is ∼10.8 times higher than the power density of the reported state-of-the-art photocatalytic fuel cell (PFC). This outstanding capability is due to the synergistic effect of the monolithic photoanode, in which the prepositive Sb/TNR generates abundant electrons and holes using short-wavelength photons, and the SC provides much higher potential than traditional PFCs to drive the electrons being transported to the cathode by absorbing the transmission of longer wavelength photons. The results also revealed that the SWENS showed remarkably stability in long-term application and is effective in clean energy production while degrading various refractory organics. This work proposed a new effective way to develop a composite water-energy nexus technology for simultaneous clean energy generation and water treatment.

Multi-functional photocatalytic fuel cell for simultaneous removal of organic pollutant and chromium (VI) accompanied with electricity production

It is of significant importance to realize the efficient wastewater treatment and energy recovery. This study presents a multi-functional photocatalytic fuel cell (PFC), which could reductively treat Cr(VI) contaminant and oxidatively degrade organic pollutant simultaneously along with electricity production in an economical strategy. TiO₂ nanotube arrays (TNA) and graphite were used as photoanode and cathode in two separated chambers, respectively. The optimized PFC with open circuit voltage of 1.06 V, maximum power density of 1 W m^-2 and short circuit current density of 3.7 A m^-2 can be obtained by increasing Cr(VI) concentration and decreasing pH values in catholyte. Under optimized PFC conditions, more photogenerated electrons will be transferred to cathode for Cr(VI) reduction, and accelerating electron-hole separation in the photoanode, then facilitating the oxidation of organic pollutants on anode. More than 96.8% removal efficiency for 6.8 mM Cr(VI) with a cathodic efficiency of 95.1% can be achieved within 6 h. Methylene blue (MB), an organic model pollutant, is totally decolorized on photoanode, which is significantly improved compare to photocatalysis (61.5% removal efficiency). The stable cycle operation of this economical PFC has obtained owing to the stable and low cost materials of both photoanode and cathode. This work may provide an efficient and economical method to simultaneously remove two types of pollutants with electricity harvested in one cell.

Strong Hollow Spherical La2NiO4 Photocatalytic Microreactor for Round-the-Clock Environmental Remediation

This work reports a moderate round-the-clock route to treating organic pollutants by utilizing a La₂NiO₄ hollow-sphere microreactor. A glycerol-assisted solvothermal route followed by an annealing process was applied for fabricating the catalyst. Both the physicochemical properties and the catalytic performance of the as-obtained microreactor for treating pollutants were discussed. The microreactor exhibited a strong ability to degrade phenol and anionic dyes in the absence of light irradiation, owing to its high surface area and positively charged surface. With the aid of visible-light irradiation, the degradation rate of the organic pollutants could be further accelerated due to the light multireflection in a hollow structure, which enhances the utilization of light. The present work indicates that the hollow-sphere La₂NiO₄ microreactor is effectively energy saving for environmental remediation.

Gas-Phase Photoelectrocatalysis for Breaking Down Nitric Oxide

Photoelectrocatalysis (PEC) produces high-efficiency electron-hole separation by applying a bias voltage between semiconductor-based electrodes to achieve high photocatalytic reaction rates. However, using PEC to treat polluted gas in a gas-phase reaction is difficult because of the lack of a conductive medium. Herein, we report an efficient PEC system to oxidize NO gas by using parallel photoactive composites (TiO₂ nanoribbons-carbon nanotubes) coated on stainless-steel mesh as photoanodes in a gas-phase chamber and Pt foil as the working electrode in a liquid-phase auxiliary cell. Carbon nanotubes (CNTs) were utilized as conductive scaffolds to enhance the interaction between TiO₂ and stainless-steel skeletons for accelerated photogenerated electron transfer. Such a PEC system exhibited super-high performance for the treatment of indoor NO gas (550 ppb) with high selectivity for nitrate under UV-light irradiation owing to the conductive, intertwined network structure of the photoanode, fast photocarrier separation, and longer photogenerated hole lifetime. The photogenerated holes were proven to be the most important active sites for directly driving PEC oxidation of indoor NO gas, even in the absence of water vapor. This work created an efficient PEC air-purification filter for treating indoor polluted air under ambient conditions.

Copper Phosphide-Enhanced Lower Charge Trapping Occurrence in Graphitic-C3N4 for Efficient Noble-Metal-Free Photocatalytic H2 Evolution

Graphitic carbon nitride (g-C₃N₄) fundamental photophysical processes exhibit a high frequency of charge trapping due to physicochemical defects. In this study, a copper phosphide (Cu₃P) and g-C₃N₄ hybrid was synthesized via a facile phosphorization method. Cu₃P, as an electron acceptor, efficiently captures the photogenerated electrons and drastically improved the charge separation rate to cause a significantly enhanced photocatalytic performance. Moreover, the robust and intimate chemical interactions between Cu₃P and g-C₃N₄ offers a rectified charge-transfer channel that can lead to a higher H₂ evolution rate (HRE, 277.2 μmol h^-1 g^-1) for this hybrid that is up to 370 times greater than that achieved from using bare g-C₃N₄ (HRE, 0.75 μmol h^-1 g^-1) with a quantum efficiency of 3.74% under visible light irradiation (λ = 420 nm). To better determine the photophysical characteristics of the Cu₃P-induced charge antitrapping behavior, ultrafast time-resolved spectroscopy measurements were used to investigate the charge carriers' dynamics from femtosecond to nanosecond time domains. The experimental results clearly revealed that Cu₃P can effectively enhance charge transfer and suppress photoelectron-hole recombination.

A chloroplast structured photocatalyst enabled by microwave synthesis

Photosynthesis occurs through the synergistic effects of the non-ncontinuously distributed components in the chloroplast. Inspired by nature, we mimic chloroplast and develop a generic approach to synthesize non-continuously distributed semiconductors threaded by carbon nanotubes. In the synthesis, carbon nanotubes serve as microwave antennas to produce local super-hot dots on the surface, which might induce and accelerate various organic/inorganic semiconductors assembly. With the unique nanoscale designed bionic architecture, a chloroplast structured photocatalyst with 3−dimentional dual electron transfer pathways facilitate enhanced photocatalytic performance. The as-synthesized carbon nanotubes-titanium oxide achieves a record-breaking efficiency of 86% for nitric oxide treatment under ultraviolet light irradiation. As a general strategy, a wide variety of carbon nanotubes threaded chloroplast structured nanomaterials can be synthesized and these nanomaterials could find applications in energy chemistry, environmental science and human health.

Inspired by the favorable configuration of chloroplast, here the authors develop a microwave aided approach for the synthesis of similarly structured photocatalysts. The non-continuously distributed TiO₂ nanomaterials threaded by carbon nanotubes enable a high efficiency for nitric oxide treatment.